This invention relates to circuit breaker panelboard enclosures and more particularly, to explosion-proof enclosures preventing the destruction of the circuit breakers contained within the enclosure during an explosion. Still more particularly, the invention relates to stress absorbing members which absorb the stress exerted on the circuit breakers by the explosion.
Explosion-proof panelboard enclosures are required in hazardous Class I (gaseous) environments to contain any explosion originating in a panelboard enclosure and reduce the risk of the explosion spreading to the surrounding environment. The purpose of the explosion-proof panelboard enclosure is to minimize the hazards of electrical arcing in areas where such arcs can cause an explosion. Therefore, a conventional explosion-proof panelboard enclosure typically consists of a cast metal box designed to contain explosions caused by the ignition of combustible gases which permeate inside the enclosure.
The circuit breakers are enclosed in the explosion-proof panelboard. Circuit breakers inherently by their operation create electrical arcing when they are triggered during high voltage operation. Circuit breakers typically contain at least one normally closed electrical contact. The circuit breaker opens the electrical contacts to protect high voltage circuitry and equipment from either power surges or electrical shorts. When the contacts begin to physically separate, the high voltage potential across the contacts arcs from one contact to the other. Arcing will continue until the physical distance between the contacts is sufficient to prohibit arcing.
Explosion proof panelboard enclosures are designed to prohibit an explosion from propagating outside the enclosure. This is accomplished by eliminating all potential flamepaths from inside the panelboard enclosure to the outside surrounding environment. Panelboard enclosures are not, however, sealed to prevent gases from permeating inside the enclosure. Thus, when combustible gas is present inside the panelboard and a circuit breaker is triggered, electrical arcing may ignite the gas. During the resultant explosion, the ignited gas sends a gas pressure wave through the interior of the panelboard enclosure pressure piling the gas inside the housing of the circuit breakers disposed within the enclosure. This pressure piling, especially with Group B gas, creates a high pressure explosion inside the circuit breaker housing and subsequently blows up the breaker.
Testing of panelboard enclosures is governed by National Electric Code (N.E.C.) classifications for areas where the use of electrical equipment can cause ambient gas to explode. In its simplest form, testing a panelboard typically consists of filling the enclosure with a combustible gas, such as a mixture of hydrogen and air, and igniting the gas with an ignition device installed in the panelboard. The resulting explosion must be contained within the enclosure so that flames do not propagate outside the enclosure, thus setting off a greater explosion. Further, the enclosure must be strong enough to withstand the explosion without breaching and the temperature of the panelboard after an explosion can not exceed a specified percentage of the explosion temperature of the gas under test, typically 80 percent. The specification for the minimum acceptable panelboard strength for a new panelboard design is calculated by multiplying the hydrostatic pressure measured during initial testing by a value pre-determined by Underwriters Laboratory, typically four (4). This strength specification dictates the type of materials that must be used to construct the panelboard and establishes the design parameters necessary to insure containment of explosions in hazardous areas.
Current panelboard explosions result in the destruction of the circuit breakers contained therein. This destruction is believed to be the result of the ignition of gas which both permeated into the circuit breaker housing and was forced into the housing by pressure piling.
When hydrogen is ignited in an enclosed chamber, a compressive wave front is created. This wave front originates at the point of ignition and propagates through the chamber proceeding the explosion. The wave front compresses the hydrogen in front of the wave, thereby increasing the density and explosive force of the hydrogen. It is believed that when the wave front reaches a circuit breaker it piles, or forces, highly compressed gas inside of the circuit breaker housing so that the gas inside the housing becomes very dense. When the explosion reaches the circuit breaker, the dense gas inside the housing ignites. The explosion of this highly dense gas in the relatively small area of a circuit breaker housing, results in a high pressure explosion which breaches the circuit breaker housing.
When the housing of the circuit breaker is ruptured, the higher circuit breaker explosion pressure is translated to the panelboard enclosure. Further, in an application where there is a plurality of circuit breakers in a row, the explosive wave front travels from one circuit breaker to the next; destroying each component. Explosions in a panelboard enclosure containing one or more circuit breakers result in an explosion pressure three (3) times higher than an identical explosion in an empty enclosure. Therefore, the destruction of a single component increases the cumulative explosion pressure in the panelboard enclosure, increasing the necessary strength specifications for the panelboard enclosure.
It is important that the circuit breakers contained within the panelboard enclosure maintain their integrity and continue to function after an explosion. The present invention overcomes the deficiencies of prior art enclosures by ensuring the integrity of the circuit breakers during an explosion.